The invention is based on a method of protecting catalytic converters for exhaust gas purification by measuring the temperature with a heat tone sensor and to a heat tone sensor for implementing the method. The method and the heat tone sensor for implementing the method are particularly suitable for use in motor vehicles.
The exhaust gas of motor vehicle internal-combustion engines includes, among others, carbon monoxide, nitrous oxides and non-combusted or partially combusted hydrocarbons which contribute to air pollution. To decrease air pollution caused by these substances to a minimum value, it is necessary to substantially eliminate these substances from the exhaust gases of motor vehicle internal-combustion engines. This is done by converting the noxious components of the exhaust gas to the non-noxious compounds carbon dioxide, nitrogen and water in that the exhaust gases are subjected to further combustion conducted over catalysts at temperatures above about 600.degree. C.
The service life of such catalytic converters is limited. This means that their catalytic activity decreases more or less rapidly after a certain time and the exhaust gases thus may reach the atmosphere in their unpurified state. Since, however, the legal regulations for exhaust gases must always be compiled with during the entire time the vehicle is in use, it is recommended to monitor the proper operation of the catalytic converter. This monitoring system must indicate to the driver by way of an optical or acoustic signal that the catalytic converter no longer operates properly.
One possibility of directly monitoring the catalytic converter is by measuring the decomposition of one or several components of the contaminants, in which case their concentration is measured once upstream and once downstream of the catalytic converter so that the difference in concentration can be used as a measure for the proper operation of the catalytic converter. However, measuring these contaminant components is relatively expensive and difficult.
German Patent 2,643,739 further discloses a method of monitoring the activity of catalytic converters for exhaust gas purification by measuring the temperature with temperature sensors. In this method, a temperature comparison measurement is made with the aid of two temperature sensors which measure the heat tone developed by the reaction taking place in the catalytic converter and, if there is a temperature difference as an indication that the catalytic converter is no longer able to operate properly, causes a warning signal to be emitted.
In the prior art method, two temperature sensors are advantageously employed for the temperature measurements. One of the temperature sensors has a catalytically active surface, the other, however, has a catalytically inactive surface. Both temperature sensors are arranged in the gas chamber in close proximity to one another, but without contacting one another, within the catalytic converter system or shortly downstream of it.